This invention relates to a high quality single crystal of a compound which is likely decomposed at a neighbor of the melting point, and a process for preparing the same.
Oxysulfides of rare earth elements represented by the formula: RE.sub.2 O.sub.2 S where RE represents at least one element selected from rare earth elements including Y have recently been attracted to attention as constitutional materials for scintillators or a laser rod. For example, single crystals such as Gd.sub.2 O.sub.2 S to which Nd is doped, La.sub.2 O.sub.2 S to which Nd is doped, etc. can effect luminescence with high efficiency as compared to the conventional YAG laser, etc. so that researches have now been made to use these crystals as a material for a laser rod. Also, single crystals such as Gd.sub.2 O.sub.2 S to which Pr is doped, etc. are expected to be used as a scintillator material of a CT scanner or a display for a color cathode-ray tube. Further, single crystals such as Gd.sub.2 O.sub.2 S to which Eu or Tb is doped, etc. are expected to be used as a scintillator material of a display for a color cathode-ray tube.
As methods for preparing the single crystals of oxysulfide as mentioned above, there have been investigated the methods such as a usual melt drawing method (e.g. Czochralski method), a zone melting method, a flux method, a vertical Bridgeman method (e.g. Bridgeman-Stockberger method) and others, and some of them has been tried. However, RE.sub.2 O.sub.2 S has difficulty because it is decomposed at a temperature around the melting point and sulfur component is likely evaporated so that the above conventional single crystal growing method involved some problems. For example, if the usual melt drawing method is applied, sulfur component is evaporated when melting or drawing so that the atomic ratio of sulfur of the resulting single crystal composition is likely smaller than the stoichiometric composition whereby it is difficult to obtain a high quality single crystal. There have been attempted to effect the melt drawing under pressurized atmosphere or apply to a melting solution capsule method, but sufficient effects have not yet been obtained.
Also, there has been considered to carry out zone melting by using a compact powder material obtained by placing RE.sub.2 O.sub.2 S powder in an airtight container and subjecting to heat treatment under inert atmosphere. However, in the case of powder, even when it is filled in a container closed-pack, its occupied volume ratio is at most 60% or so whereby sulfur component is evaporated to a space formed during partial melting of the powder. Thus, similarly in the above melt drawing method, quality of a single crystal is lowered.
Contrary to the above methods, in the flux method, if a suitable flux can be found depending on the objective composition, growth of a single crystal can be carried out at a temperature 50 to 60% of the melting point in terms of the absolute temperature whereby evaporation of sulfur component can be prevented and high quality single crystal can be obtained. In fact, there has been reported that high quality single crystal of La.sub.2 O.sub.2 S could be obtained by using K.sub.2 S as a flux (see Proceedings of 12th Rare Earth Research Conf (1976)). However, the size of the single crystal obtained by the flux method is at most several mm or so in diameter. Even when a material having the size exceeding the above can be obtained, the obtained material is an agglomerate of these fine single crystals so that it cannot be practically used for industrial purpose.
On the other hand, there has been reported that relatively high quality single crystal of RE.sub.2 O.sub.2 S with a larger size than that of the flux method can be obtained by the vertical bridgeman method (see JOURNAL OF APPLIED PHYSICS, Vol. 42, Number 8, July (1971)). In the vertical bridgeman method, when crystal is grown in, for example, an Ar atmosphere at 10 kgf/cm.sup.2, a composition at a melted portion will change with a lapse of time and thus, even if a good single crystal with a stoichiometric composition can be obtained at an initial stage, sulfur component gradually lacks in the composition. Accordingly, a volume of the single crystal with high quality is a little and a single crystal having a practically usable size cannot yet be obtained.
As described above, whereas an oxysulfide of a rare earth element represented by the formula RE.sub.2 O.sub.2 S has been attracted to attention as a constitutional material for a laser rod or a scintillator with high efflciency, it has a problem that RE.sub.2 O.sub.2 S itself is likely decomposed at a temperature around the melting point and sulfur component is likely evaporated. Thus, it has a problem that high quality single crystal having a sufficient volume which can be applied to an industrial use and has a composition substantially the same with a stoichiometric one cannot be obtained only by applying the conventional single crystal growing methods.
Also, the problem of preparing single crystal caused by evaporation of a component at a temperature around the melting point occurs not only in RE.sub.2 O.sub.2 S but also, for example, in preparation of REVO.sub.4 single crystal which is expected to be applied to a laser rod.
Thus, for preparing a single crystal of a compound in which at least part thereof is likely evaporated at a temperature around the melting point, it has been strongly demanded to maintain quality of the single crystal by depressing evaporation of a component and to enable preparation of a single crystal having a practical size used for an industrial use.